- transform html page into responsive, modern notepad application

- implement dark mode with persistent theme preference
- add rich text formatting with `execCommand()` (bold, italic, underline, lists)
- improve note management with sidebar and searchable note list
- add word/character counter in editor footer
- implement autosave functionality with 30 second interval
- create proper undo/redo stack for editor content
- add import/export functionality with JSON file support
- fix search with highlighting and result navigation
- implement keyboard shortcuts (`Ctrl+S`, `Ctrl+Z`, etc.)
- add toast notifications for user feedback
- implement confirmation dialogs for destructive actions
- improve UI with proper CSS variables and responsive design
- fix content saving to preserve user input

Author: TobiasMue91